R&D work conducted under the aegis of EBU (European Broadcasting Union) has led to defining a direct satellite television broadcasting system in the 12 GHz band, called "MAC-PACKET" a description of which can be found in the paper "The C-MAC/Packet system for direct satellite tv (EBU Review - Technical No. 220, August 1983). This system uses a RF time division multiplex signal in which the carrier is modulated by analog image signals (luminance and colour-difference) for a fraction of one scanning line and digitally for another fraction of the line duration by data (sound, synchronization, data channels if necessary). The signal may be such as shown in FIG. 1 in which the digital elements are transmitted in a multiplex by packets. In FIG. 1, the successive periods are assigned in the following way:
a: period occupied by a burst of digitally coded data, preceded by a line sync word,
b: transition from end of data, including the leading edge of the separation (pedestal) signal added to the video to provide energy dispersal (this signal being possibly a triangular wave synchronous with the picture),
c: clamping period (giving the zero level of the color difference and allowing the signal to be aligned at each scanning line),
sc1: reserved for video encryption or scrambling,
d: colour difference (chrominance) as analog samples with time compression in a 3/1 ratio,
g: luminance, as analog samples with time compression in a 3/2 ratio,
sc2: reserved for video encryption or scrambling,
h: transition to data, including the trailing edge of the pedestal signal.
The C-MAC packet system now proposed for direct satellite television broadcast in the 12 GHz channel uses four state phase shift keying (MDP 2-4 or 2-4 FSK), at a clock frequency of 20.25 MHz, which leads to a frequency spectrum requirement incompatible with the use of the same coding as in land based networks. In addition, 2-4 FSK modulation (which uses digital coding with whole response) involves compliance with the first criterion of Nyquist, so a -6 dB passage point at the symbol half frequency has a high sensitivity to high frequency distorsions. Finally, the absence of base band representation further raises problems difficult to overcome when transmission takes place over land carriers (wired networks, radio links) using frequencies in the 7-8 MHz range.